For example, in order to attain stable combustion of a lean air-fuel mixture in a spark ignition type internal combustion engine, gas flow within the cylinder such as tumble or swirl (lateral flow) is extremely important, and such gas flow should be strengthened in a wider operation region.
Particularly in a low-load region among the operation regions of the internal combustion engine in which a throttle opening is small and an intake air quantity is correspondingly small, generally, an air-fuel mixture is made slightly richer in order to stabilize combustion. Accordingly, fuel efficiency or emission tends to deteriorate. In order to improve fuel efficiency or emission, it is effective to generate whirls of intake air within the cylinder, to promote combustion by using strong turbulent flow. That is, tumble or swirl is generated in the intake air.
Here, the swirl refers to flow of the intake air along a circumferential wall of the cylinder. The swirl is effective in homogenizing the intake air, whereas less effective in promoting combustion by generating turbulent flow. On the other hand, the tumble refers to flow of the intake air along an axial direction of the cylinder. As the tumble breaks up in a latter half of compression stroke, strong turbulent flow is generated. Therefore, the tumble is effective as a measure to improve combustion in a low-load region of the engine.
An example of such a method of strengthening gas flow (swirl and tumble) within the cylinder includes a method of using an intake control valve partially blocking a cross-section of a passage of the intake port to cause intake air flow within the intake port to be present locally on one side of the intake port. For example, in order to generate tumble, an intake control valve is disposed in a lower portion of the intake port, so that the intake air flows locally through an upper portion of the intake port. The tumble within the cylinder is thus strengthened.
That is, when gas flow is strengthened, the cross-sectional area of the passage of the intake port is substantially made smaller by the intake control valve. Here, a ratio of the cross-sectional area of the passage effective with respect to a reference intake port cross-sectional area is defined as “opening ratio.” Generally, the smaller the opening ratio is, the stronger the gas flow is. On the other hand, if the opening ratio is small, fluid resistance is increased and a quantity of intake air that can be taken into the cylinder is decreased. Accordingly, an operation condition allowing strengthening of gas flow by closing the intake control valve is limited to a relatively narrow range. Japanese Patent Laying-Open No. 2004-124836 discloses an air intake apparatus for an internal combustion engine capable of strengthening gas flow within a cylinder without excessively lowering the opening ratio. The air intake apparatus for an internal combustion engine in which an intake port is connected to a cylinder of the internal combustion engine and an intake valve opens and closes a downstream end of the intake port includes a partition wall provided along a longitudinal direction of the intake port so as to cross-sectionally partition the intake port into two sections, an intake control valve located in proximity of an upstream end of the partition wall and opening and closing one flow path implemented by partition by the partition wall, and a communication path for communication between two flow paths implemented by partition by the partition wall at a position close to the intake control valve.
According to the air intake apparatus for the internal combustion engine, when the intake control valve is at a closing position for blocking one flow path, the intake air flows toward the cylinder side only through the other flow path, so that a relatively large quantity of intake air flows into the cylinder through a portion around the intake valve closer to one side. At the same time, the intake control valve narrows the intake air flow, to cause local pressure lowering on the downstream side of the intake control valve, which in turn influences on the communication path. As such, a pressure difference is generated between the downstream end of one flow path blocked by the intake control valve and the communication path, intake air is suctioned from the end portion and flows backward to the upstream side of the intake port, and the intake air merges into the other flow path through the communication path. In other words, a part of the intake air returns to the upstream side via the blocked flow path. Then, flow rate or flow velocity of the intake air flow passing a portion around the intake valve is further unbalanced, so as to effectively strengthen gas flow within the cylinder. Consequently, according to the air intake apparatus for the internal combustion engine, a part of the intake air can return via the flow path blocked by the intake control valve, so as to effectively improve gas flow within the cylinder. Particularly, stronger gas flow can be obtained without lowering the opening ratio by means of the intake control valve. Therefore, increase in pumping loss due to increase in fluid resistance can be suppressed and a larger quantity of intake air flowing into the cylinder can be ensured, so that gas flow can be strengthened in a wider operation region.
On the other hand, in the air intake apparatus for the internal combustion engine disclosed in Japanese Patent Laying-Open No. 2004-124836, a communication path for returning a part of the intake air to the upstream side via the blocked flow path should be provided in the partition wall. Provision of a communication path causes a further complicated structure of an intake manifold including the intake control valve. In addition, a partition wall should be provided in the air intake apparatus for the internal combustion engine disclosed in this publication. Provision of the partition wall greatly contributes to strengthening of a current, however, causes further complicated structure of the intake manifold including the intake control valve.